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How do pickups sense the sound?

In most electric instruments the string ripple is sensed by magnetic pickups. These pickups usually consist of magnets, coils, core materials and/or polepieces.

The magnetic field of the magnet links the elements of the pickup (magnets, coil, cores, polepieces, etc.) and the string.

Changes in the string position cause small changes in the total magnetic flux linking the pickup elements and the string. As the string gets closer to the magnets or polepieces, the total magnetic flux linking the elements of the pickup increases. The pickup coils produce electrical pulses that correspond to changes in the magnetic flux that passes through the coils. When the string moves suddenly from position (a) to position (b) and stays there, the voltage signal at the coil ends rises and then settles back to zero. Together, the pickup coil and its core materials are a real inductor, meaning that it has some capacitance, it is not a pure inductance. Because of the capacitance, which comes from the proximity of each wire turn to the others, the real inductor will have a resonance.

Its impedance will show a peak (maximum) at that resonant frequency. The resonance of the pickup coil can enhance some harmonics of the note being played and change its tonality.